The overall objective of this research project is to characterize the processes involved in the physiological regulation of thyroid gland function. Studies are being performed to determine the mechanism of the thyroid iodide transport process, the nature of its coupling to sodium transport and finally its regulation both by thyrotropin and iodide itself. Studies are performed in isolated beef thyroid cells, prepared by trypic dispersion of minced beef thyroid glands, and fetal and neonatal cat thyroid glands. Another major area of research is concerned with the study of the action of thyrotropin on thyroid cell metabolism, especially cyclic AMP accumulation, iodide metabolism and protein synthesis. These experiments are designed to investigate the affect of other physiological active substances such as iodide ion, epinephrine and insulin in modifying the responsivity of the thyroid cell to thyrotropin stimulation. In addition, these studies will be used to investigate the relationship between the thyrotropin concentration, intracellular cyclic AMP concentration and the physiological response of the tissue. These studies will lead to a better understanding of the regulation of intrathyroidal iodide metabolism which is essential for th elucidation of the processes leading to the goiter of iodine deficiency, and endemic cretinism.